The usually cordial relationship between William Ackman and Target Corp. has taken a more combative turn, as the Pershing Square Capital Management chief has launched a proxy battle with the retailer.

Pershing Square, which owns an 7.8% stake in Target and manages a hedge fund that invests solely in the company’s securities—and which has lost more than 90% of its value since it launched two years ago—is offering five director nominees, including Ackman himself. For its part, Target said it was “disappointed” and said it had frequently engaged with Ackman and Pershing Square.

But Ackman has been unable to convince Target management to make moves he said would help boost its stock price, including spinning off its real-estate holdings into a separate real-estate investment trust. Still, Ackman says he’s not looking for board representation to oust the Minneapolis company’s management.

“We thought Target would be where Wal-Mart has been during these very difficult economic times,” Ackman told Bloomberg News. “This is not your classic proxy contest. This is a management team we have a very high regard for.”

For Target, however, that endorsement is cold comfort.

“We are disappointed that Pershing Square has decided to pursue a costly and disruptive proxy contest, especially in light of our previous dialogue,” Target said in a statement. “Target has a long history of being responsive to shareholders and has engaged in numerous discussions with Pershing Square over a 20-month period.”